Asymptotic
by define-serenity
Summary: [Barry/Caitlin] "The multiverse theory of quantum physics," Doctor Wells' voice echoes through the auditorium, the room too big for them, but it's the one auditorium the school could spare at this time for Dr. Wells' Lucky Fifteen. "Simply put, it posits the existence of an infinite number of parallel universes that exists concurrently with our own."


**author's notes:** written for simplysnowbarry's **Throwback Snowbarry** challenge, day 2: What Could Have Been, using the prompt 'Other Earths.'

 _._

 **Asymptotic**

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 _Earth-1_

Her heels click down the ramp at a soft and even pace, Barry's, "Some people are worth being crazy for," breezing past her shoulder.

Yes, there are people that came into her life who were worth risking it all for, who made every moment she stepped outside of her comfort zone worthwhile and precious. Like Ronnie did. Like Barry had last night, when a few drinks too many landed her on stage singing karaoke.

"I'm sorry if I was a bit of a drunken mess the other night," she says, fingers wiring together unlike the fragments of the night before; she can't remember much, and she thinks it a pity, because she and Barry make quite the pair right here at the lab and it would've been interesting to see how they worked as co-workers friendly enough with each other to go out for drinks, without that having to mean anything more.

"Actually" -Barry chuckles- "It was pretty fun."

"Yeah." She beams, what little she does remember warming her chest. "It was."

At the end of the night, Barry'd made sure she made it home okay, helped her change, tucked her in, and stayed with her until she fell asleep; there aren't many men who would've done that without expecting something more in return.

In the hallway Barry slows to a standstill, and she keeps pace with him, especially once he utters the words, "You know what? I think everybody's been right about you and me," and she can't help but wonder what everyone's been saying. She and Barry might share more tragic stories, might have found a kinship born from that same tragedy, but there's little about that past within their grasp to change.

"Both of us have been hung up on people for way too long," Barry says, hands in his pockets.

Her shoulders, previously awkward and tense, relax a little, and she thinks about her sweet goofy Ronnie, about her unexpected hero whose ghost she's been living with within these walls. 'Hung up' may not be the right term, but she's been reluctant to pass to the acceptance phase of her grieving process. Maybe that's because Barry's showed her the impossible isn't without merit. But that doesn't mean she disagrees with Barry. She's been stuck for more than nine months.

"If what Cisco says is true, that Ronnie merged with Martin Stein, then—" Drawing in a deep breath, her eyes go out of focus. Ronnie could be stuck too, fighting his way back to her, but she's all too aware that's the kind of wishful thinking that grounded her in the first place, "—he's not alive anymore."

Like always, Barry gives her words enough space to breathe, to find a setting she can accept as her new normal; Ronnie's gone, and that's not anyone's fault, and she can't keep living her life in service of that loss. If last night showed her anything it's that she's ready to make a change, rediscover a part of herself that'd gotten lost in the storm.

A small smile pulls at her mouth. "Time for me to move on," she says, content that it's the right choice for her — she's too young to give up on love, and she wasn't made to be alone. Who knows, the right person could be right around the corner. "Find someone new to be crazy about."

Her eyes find Barry's, an identical twinkle in them, as if he's thinking the same thing. He can't keep holding out for Iris, and he shouldn't get stuck on the idea that his lifelong friendship will someday win her over; that's how resentment started, and it'd be a shame to see their friendship sour simply because Barry can't broaden his horizons. There's nothing stopping him from calling that girl who gave him her number, or just have fun and go on lots of dates.

There's nothing stopping her, either.

With Barry's eyes lingering on her face, the faint memory of him watching over her as she fell asleep heats up her cheeks. She'd been far too forward and flirty, yet it hadn't deterred Barry from taking care of her, or teasing her about it this morning.

But if she were to put herself out there again, Barry would be a terrible place to start, what with his penchant for running into danger rather than away from it; she's drawn to that heroism, no doubt about it, but as romantic as the idea of a relationship grounded in friendship sounded, she can't be hurt like that again.

It's time for her to move on, just not with Barry.

There must be someone for her, out there, in the great big somewhere.

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 _Earth-15_

"The multiverse theory of quantum physics," Doctor Wells' voice echoes through the auditorium, the room too big for them, but it's the one auditorium the school could spare at this time for Dr. Wells' Lucky Fifteen, an affectionate nickname their group got following the exclusive selection process they all went through. If Dr. Wells could be trusted, and his opinions were held in high regard by the faculty, their small group represented the best and brightest, with the biggest future ahead in their respective fields.

"Simply put" -Dr Wells points at someone in the front row- "it posits the existence of an infinite number of parallel universes that exists concurrently with our own."

Dr. Wells had made the theory of the multiverse one of his major fields of study, along with particle physics and quantum mechanics; it held that anything that can happen, will happen, in one reality or other. As a field it was one of the more contested ones, because since its earliest known mention in 1952 its existence had yet to be proved — not that that had ever dissuaded scientists from hypothesizing. Dark matter and exotic particles were all science fiction too, but that hasn't stopped people from researching it.

Yet, while Dr. Wells was one of the most brilliant men she'd ever met, the thought that there might be more Caitlin Snows out there, that there could be girls with her face living in realities running parallel to hers felt like too much to wrap her mind around. She'd seen things right here in this class that defied definition, but parallel worlds? Alternate universes? She has enough trouble wrapping her mind around one of her — she couldn't worry about any other theoretical hers too.

As Dr. Wells' voice fades into the background, Caitlin can't help but wonder: could it be? Could there be Earths where she exists differently? Where she's not a PhD student? Where she doesn't wear big geeky glasses she can't afford to replace and she's not on anti-anxiety medication to deal with the pressure she -and her dad- put on her? Could there be a world where she and her dad get along? There are decisions she regrets making, and character traits she wishes she could change — it's almost comforting to think that somewhere, out there, she might exist exactly the way she wants to.

"Anything we can help you with, Miss Snow?" Dr. Wells asks, sending heat to her cheeks and an acute awareness of how ten heads in the room turn her way. All but one.

"She's just wondering if there are any alternate Caitlins enjoying your class right now, Dr. Wells," Barry quips, sending a wave of laughter through the auditorium that shifts the attention away from her, and she bites at her lower lip.

Her eyes catch her boyfriend's, her sweet goofy Barry, both of them unable to stifle a smile. Barry winks at her, and she wishes he was within reach so she could hug him, but there's a reason their seats are on opposite sides of the aisle; they've already had three wonderful years together, and Barry's still the biggest distraction to her whenever he's close — sitting together in class stopped being an option and studying by themselves was all but out of the question. Luckily they had friends happy to come between them.

"You think there's a universe out there where Allen doesn't stick up for our dear Caitlin?" another student, Hartley Rathaway, remarks, making sure her cheeks flush even hotter.

It's a thought almost too unbearable to consider; on what possible Earth would Barry Allen not be the boy of her dreams?

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 _Earth-52_

"Dream on, _Flash_."

She spits his name with that crisp white grin rounded by the darkest lips, contrasting so deeply with her pale skin he almost mistakes them as part of the night sky. With Deathstorm by her side she stands even taller, hands shaped into claws, an ominous mist creeping over the ground towards them.

"You're no match for the both of us," she sneers as Ronnie Raymond steps up next to her; his fire meets Killer Frost's ice, making for one of the deadliest combinations Central City has thus far experienced. Deathstorm never says much, battling the other voice inside his head, but he would follow his bride to hell and back.

"Give it up, Frost."

His own bride joins him for the very first time, the red of her suit a perfect match for his. She'd follow him into oblivion too, into the Speed Force and beyond, into the past and the future, even if it meant creating paradoxes that could bring this world to its knees — they're a package deal as surely as Killer Frost and Deathstorm and they all knew this couldn't last; whatever precarious status quo he achieved won't withstand Killer Frost's hunger for heat.

"You're surrounded," Iris says, her eyes fierce inside the mask sat snug across her face, and as if on cue police sirens blare, Captain Singh and Eddie and the rest of the CCPD coming to their aid. Armed with guns and modified shields a small army of police officers moves in, locking the four of them into a single street block. Whatever happens, this ends tonight. They all know that.

Killer Frost's gone too far this time. She killed too many innocents, too many too close to his heart.

"It's my lucky day" -Killer Frost shrugs, unaffected by the force brought to bear against her, and her mouth sets in that animalistic snarl he's caught too many times to count- "I get to kill two Speedsters with one stone."

Without warning a cold blast blows them several feet back, but he and Iris recover quickly — they're meant to, he understands how this works. Killer Frost likes to play with her victims before going in for the kill, but once she does nothing will stop her from draining each molecule of warmth out of them.

"How is daddy Joe dealing with the death of Kid Flash?" she asks, a dangerous playfulness dancing around her stone cold irises.

"Don't _you dare_ say his name." Iris shoots forward, and Deathstorm takes his shot, counting on Iris to lose her cool and fires all he has, but Iris anticipates the move and starts running circles around him; together it creates a funnel of fire, like he told Iris it would — before long it'll wear Deathstorm down and they can combine efforts to take out Killer Frost. There are two cells in Iron Heights waiting for them.

"Guess it's just you and me, Flash." Killer Frost turns to him, none too worried about her man. "Are you ready to find out which one of us is stronger?"

"I'm not a killer."

Killer Frost pouts, in that same old condescending manner she has, and he wonders how much longer they'll be playing this game, how many more years of his time he'll sacrifice to chasing her down.

"Well, I am," she says, her eyes darkening before her lips slip into a smile again, "and before tonight is over I will have tasted your Speed."

Before he can reply a blast wave knocks them both aside, and panic starts his otherwise calm heart beating in overdrive; his eyes fall to the motionless figure on the ground, Deathstorm already recovering. No. They had a plan.

"Iris," he breathes, and struggles upright, but slips on a patch of ice beneath his feet; he'll never make it to Iris in time, he'll never reach her before Deathstorm delivers a deadly blow, before—

Two gunshots pop his eardrums, ringing out discontentedly in the chaos around them. The bullets hit Deathstorm center mass, taking him down without remorse.

He looks to his right.

Eddie lowers his gun to the ground.

And he releases the breath he'd been holding.

"Ronnie!" whatever's left of Caitlin Snow cries out, and she rushes to her husband's side, the same way he speeds over to Iris and kneels down next to her to make sure she's okay, that he hasn't lost her too, that Killer Frost hasn't left him with another scar.

Iris stirs awake.

Ronnie Raymond does not.

"Baby?" Killer Frost whines, hands drawing through Deathstorm's hair, down his chest, her dark lips drawing to his forehead.

He stands up, weak in the knees. He hadn't meant for this to happen. No matter the metas he's faced over the years he's never killed, he never could, too afraid to turn into the monster that killed his father, or the killer that took Wally away from them, the one that broke Joe beyond repair.

He never meant for that pain to become Killer Frost's too.

For a moment or two Killer Frost stumbles around helplessly, staring between him and Iris, past them towards the police barricades, wobbly on her heels like she's become untethered.

Until her eyes fall to Eddie.

" _You_." She grinds her teeth, and a layer of ice spreads over the ground in broken patches, like it's reaching for the detective that cut Ronnie down without a second thought. "You killed him!"

He hears himself call, "Frost!" before he's running, before he accounts for the added distance the slippery underground demands, before he makes the split second decision that Killer Frost won't harm another innocent if he has anything to say about it, and phases a hand straight through her chest.

Killer Frost gasps and tries to draw in a breath, but she fails, dropping to her knees where she stands. Sinking down to the ground he takes her in his arms, whispering, "I'm sorry," knowing the fate that awaits her. How had things spiraled so out of control? How did they get here?

It's hard to remember where it even started. This cat-and-mouse game started long before Killer Frost found Deathstorm to stand by her side and long before he revealed his double life to Iris, long before either of them had any clue about the other's true identity.

Long before he made the mistake of thinking there was a wounded girl behind the cold persona she adopted.

A smile curls around Killer Frost's lips. "Now you're a killer too."

His eyes close. He never meant for this to happen.

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 _Earth-27_

"Caitlin," he breathes her name as if to revere it, like he's revered her body and soul, and his trembling hands cup her face as if he could break her any worse.

She sniggers, "You haven't called me that in years", before she starts to cough up blood that taints her pale skin a fiery red. Her wounds are too severe for her body to heal, and there's nowhere for them to go for help — someone close betrayed them, let in a snake who caught them unaware and she's the one paying the heaviest price.

It was Joe West's final pitiful attempt at avenging his children, unwittingly starting a chain of events that will send him back to the past, that will lead him to kill Iris, and break both Wally and Barry Allen to pieces.

"It's okay, baby." Killer Frost reaches up for his face, touching his scars as gently as the first time she had, so many years ago, and even manages a smile. "You'll see me again."

He will. He'll find her in every moment of his past, every minute of his becoming, and every second of hers, but even in that knowledge their goodbye becomes no more bearable. This day was coming no matter what, time had never worked in their favor, but he never imagined it would be anything like this. He never imagined he'd lose her before he left her.

"Do me a favor," Killer Frost chokes out, writhing in pain; if it were up to her she'd never show it, but she's at the unfortunate disadvantage that he reads her every micro-expression.

"Anything."

Her eyes flash white, and she grits her teeth together. "Burn this Earth to ashes," she says, and in one final effort pulls him down to bring their lips together — he kisses her as if the world were ending, as if the two of them together are some romantic bookend to an epic love story and losing each other after everything they've been through seems utterly unfair. Pulling away and bringing their foreheads together he decides once more there's never been anything fair about this; her friends cast her aside the moment she gained powers, and he got rejected as some weak copy of Barry Allen not worth loving.

But God, had they loved each other all the more because of it.

Theirs may not be a traditional love story but it's one for the ages, distorted and unchronological, absurd almost, defying the laws of physics themselves. It suited them, and neither of them had for one second backed down from it.

"Caitlin," he whispers to her lips; they're colder than he's ever felt them.

On this Earth, too, some version of Caitlin Snow dies in the arms of a man who once listened to the name Barry Allen.

She called him by another name.

Savitar.

He's a God among humans and metas alike, and he gives his queen exactly what she asked for; he burns the very ground he buries her in, he scorches the Earth beyond recognition with his vengeance, murders and maims until it runs red with blood. He tears down houses with families inside, overflows rivers and ignites long dead volcanoes anew.

If he could he'd explode the sun and take the rest of the galaxy down with him.

All for her.

Then, he keeps his promise to Joe West.

He travels back in time and creates himself, scatters mentions of his name all through history and the multiverse, gathers followers and acolytes and a nickname or two. He becomes exactly what his queen told him he would become; faster than any other meta, feared and worshipped at the same time, and Barry Allen's worst nightmare come to life.

Caitlin Snow, he watches from afar, struggling, fighting the growing cold inside her, and her pathetic dalliance with Julian Albert. When he closes his eyes he sees Killer Frost, right there by his side, beautiful and wicked, and there's a sorrow in his heart he didn't think himself capable of anymore — a God isn't meant to feel pain, but there it is anyway, aching for the Goddess she's yet to become.

He's patient, like he knew he'd have to be, but the day finally comes when he finds her stumbling alone through the snow, betrayed by one she thought she'd love, looked on as a monster by everyone else. Except for him.

"You want to cure me?" Killer Frost asks, and he barely contains a smile; that spitfire anger's already there in its rawest form, and he can't wait to see her fine-tune it. "Turn me back into Caitlin Snow?"

"No. I want to make sure Caitlin never returns."

This makes her pause for a moment, but she's been made a lot of promises of late, and he doesn't expect her to take his word for it. "Why should I trust you?" she spits, about ready to run away and hide, figure this all out on her own, fend for herself in a world that will reject her as surely as it will him.

So he bows at her feet, a thing she'll take far too much pleasure in seeing in the years to come, and unlocks his armor, takes down the walls a future version of her has successfully broken down, and reveals himself to her.

Her beautiful eyes go wide, jaw a little slack, and she stares at him in awe for the first and last time.

"What do you want me to do?"

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 _Earth-33_

"Just—" Caitlin gasps, the tilt of Barry's hips against hers starting an itch near the base of her spine, "—keep doing what you're doing."

Barry huffs a bit of an uncomfortable laugh, but listens to her directions, burying his face in the crook of her neck as he continues to thrust inside her. It's their first time together after all but three dates, and it's all a bit uncoordinated and slow, a little awkward and not very good, but Barry holds her close and that's all that matters.

Because she fell in love with him after he kissed her goodnight on their first date and he nearly tripped and fell trying to get back to his car, and he fell for her the moment he laid eyes on her; their friends set them up on a blind date, and never imagined they'd actually hit it off.

She tells him things she never imagined telling anyone, about her dad's illness and her brother's death, and he'll listen without question, without hesitation, and dries her tears if need be.

For whatever reason they're lucky to have found each other, and they've both been hurt enough in the past to recognize a good thing once they have it.

So they don't waste time on the little things, or waiting until the time is exactly right to take the next step in their relationship — together, as a couple, they're a bit uncoordinated and awkward too, but it's what makes it all so exciting.

.

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 _Earth-39_

They're not that lucky on every Earth.

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 _Earth-40_

"It's your lucky day, Mrs. Allen," Barry says over the phone, weaving through foot traffic with his long uncoordinated limbs, looking to his right and his left before crossing the street. "I had to visit five different grocery stores but I found some plain old vanilla ice cream."

Patty's laughter sounds over the line. "People say it's boring, but I happen to think it's an actual legitimate flavor."

"Alright, calm down."

"Are you coming home?" Patty asks. "To your girls?"

"I'll be there in twenty," he says, and he can vividly imagine her drawing a loving hand over the swell of her stomach, cherishing the life growing inside her, and his heart grows at the simple yet frightening thought of it. He's going to be someone's dad soon. Does he even know how to do that?

Lost in thought, he knocks shoulders with a total stranger in passing.

"Sorry," Barry mutters, unaware of the woman whose life he briefly touches, because his eyes fall to the bouncy toddler holding her hand, all curly dark hair and a dimpled smile, and he suddenly wonders if his daughter will have Patty's eyes.

"Excuse me," Caitlin apologizes to the man she runs into, but he's past her and gone before she gets a decent look at his face — her daughter tugs at her arm and points at the toy store window, the incident forgotten the moment she catches her little girl's beaming smile.

In this world, separated from Earth One by nameless degrees, Caitlin loses Ronnie, the love of her life, to a terrible tenement fire after his oxygen tank gives out, and if it hadn't been for their one-month old daughter she'd have lost herself in her grief. Barry watches Iris, the girl he loved since childhood, fall in love with Eddie Thawne, move in with him, and change her name to Iris West-Thawne not long after; luckily he meets his spitfire Spivot before he gives up on love altogether.

Their daughters, Charlotte and Nora, will meet at college many years later, both top of their class, the best and brightest in their respective fields of study.

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 _Earth-7_

There's one Earth, exactly one, that's so unbearably close to Earth Prime it's near indistinguishable, and where the potential of them lives in between each separate timeline. It's all about possibilities and making the right choices, but fate keeps pushing them together every chance it gets.

In the hallway Barry slows to a standstill, and she keeps pace with him, especially once he utters the words, "You know what? I think everybody's been right about you and me," and she can't help but wonder what everyone's been saying. She and Barry might share more tragic stories, might have found a kinship born from that same tragedy, but there's little about that past within their grasp to change.

"Both of us have been hung up on people for way too long," Barry says, hands in his pockets.

Her shoulders, previously awkward and tense, relax a little, and she thinks about her sweet goofy Ronnie, about her unexpected hero whose ghost she's been living with within these walls. 'Hung up' may not be the right word, but the acceptance phase of her grieving process had only really started once Barry woke up from his coma. She'd been stuck for nine months, until Barry showed her the impossible was real.

"If what Cisco says is true, that Ronnie merged with Martin Stein, then—"

Drawing in a deep breath, her eyes go out of focus.

If Barry could survive getting struck by lightning, surely broken hearts could mend.

"—he's not alive anymore."

Like always, Barry gives her words enough space to breathe, to find a setting she can accept as her new normal; Ronnie's gone, and that's not anyone's fault, and she can no longer live her life in service of that loss. If last night showed her anything it's that she's ready to make a change, rediscover a part of herself that'd gotten lost in the storm.

A small smile pulls at her mouth. "Time for me to move on," she says, content that it's the right choice for her — she's too young to give up on finding love, and she wasn't made to be alone. "Find someone new to be crazy about."

Her eyes find Barry's, an identical twinkle in them, as if he's thinking the same thing. His feelings for Iris had changed these past few months, and he'd started to see the merit in letting go of the past and moving on with someone else. There's nothing stopping either of them from going out and having fun.

With Barry's eyes lingering on her face, the faint memory of him watching over her as she fell asleep heats up her cheeks. She'd been far too forward and flirty, yet it hadn't deterred Barry from taking care of her, or teasing her about it this morning.

"Maybe—" Barry starts, stuttering all of a sudden, "If you'd like—"

Her heart skips a beat thinking about what Barry might suggest, because she's all too aware that a part of her is drawn to the hero in him, to his selfless surrender to a new role as The Flash, and the idea of a relationship grounded in months of friendship sounds so appealing she almost blurts out 'Yes' right away, despite not having heard his question yet.

"We could go out sometime," Barry says.

Surprisingly, she manages to school the smile threatening to overtake her face; she doesn't want to seem too eager, or rush into this and chase Barry away — this is something new for both of them and it's meant to be cherished, respected, given time and space to unfold.

"I'd like that."

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 _Earth-15_

"Earth to Dr. Snow," Barry's voice sounds at her ear all of a sudden, and she's more than a little bothered that she seems to have gotten lost in thought again during one of Dr. Wells' exclusive lectures.

She snaps to, and starts gathering her books.

"Where did you go just now?" Barry stands to his feet, curious to hear where her mind drifted off to when Dr. Wells had spoken about such exciting things.

Caitlin shrugs, following her boyfriend down the aisle and down the steps of the auditorium, out into the hallway where the usual bustle of the university greets them.

"Contemplating the intricacies of the infinite meta-verse." She shrugs with a small smile, and intertwines her fingers with Barry's.

This wasn't one of her favorite lectures; the idea of a multi-layered universe was a far too daunting one and too much to live up to, especially when living up to her dad's expectations in this one universe proved insurmountable. She can't worry about other Caitlin Snows, even if some of them may live her perfect life.

Barry, however, seemed to have taken a liking to the subject. "You think there are any other Barry Allens out there lucky enough to be dating you?"

Pursing her lips, she rises on her toes. "I don't know," she muses, and steals a sweet kiss from her boyfriend's lips. "I only care about this one."

.

.

("I love you," whispers Barry Allen to Caitlin Snow on Earths numbering near infinity, and on some she replies with an, "I love you too," right away. On some she rolls her eyes and lovingly calls him an idiot, and on yet others she punches him in the chest, because what took him so damn long?)

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 **fin**

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End file.
